The heat shock protein 70 (Hsp70) family members are powerful proteins with major roles in malignancy, such as inhibition of apoptosis, induction of resistance to chemotherapy and regulation of the stability of onco-proteins. Specifically, Hsp70 expression blocks apoptosis at several levels, and in this respect the chaperone inhibits key effectors of the apoptotic machinery, and also facilitates proteasome-mediated degradation of apoptosis-regulatory proteins. The contribution of Hsp70 isoforms to tumorigenesis is mainly through their role as co-chaperones of heat shock protein 90 (Hsp90), a heat shock protein known to regulate the transforming activities of several kinases and transcription factors. In this process, Hsp70 initiates the association of the client protein with Hsp90 through a bridging protein called HSP-organizing protein (HOP). These biological functions propose Hsp70 as an important target whose inhibition or downregulation may result in significant apoptosis in a wide-range of cancer cells, and also in inhibition of signaling pathways involved in tumorigenesis and metastasis. Due to these functions it is not surprising that Hsp70 is frequently overexpressed in cancer, where the elevated expression is furthermore believed to be a cause of resistance to chemotherapy and other treatments. These dual roles of Hsp70 in cancer, i.e. co-chaperone of Hsp90 and antiapoptotic molecule, suggest that inhibition of Hsp70 may offer a valuable anticancer strategy, as supported by Hsp70 knockdown studies.
Much effort has recently been dedicated towards the discovery of Hsp70 inhibitors and, unsurprisingly, molecules from a number of chemical classes have been reported to interact with Hsp70 through a variety of modes. These previous efforts have focused on either directly competing with ATP, which achieves potency but limited cellular activity, or by allosteric mechanisms. While these molecules are reported to elicit their effects through an Hsp70 mechanism, it is possible that they also act on multiple other unrelated and as yet unspecified mechanisms. Furthermore, these molecules tend to be hindered by a non-tractable SAR with subtle changes resulting in drastic changes in activity.
Hsp70 has proven to be a more difficult target to drug than Hsp90, which can be attributed to a number of reasons. Unlike Hsp90, there are no drug-like natural products for which Hsp70-bound crystal structures are available to guide drug design. In addition, the nucleotide binding pocket of Hsp70 is considerably more hydrophilic compared to that of Hsp90, requiring ATP to bind in a more extended conformation with polar contacts deep within the binding pocket. Reversible competitive inhibitors are particularly challenging to develop because of Hsp70's high affinity for ADP and high intracellular concentrations of ATP. While Hsp90 has proven highly amenable with numerous small-molecule ATP-competitive inhibitors entering into the clinic, to date no Hsp70 inhibitors have entered clinical trials.